Java Lab Documentation

2D Arrays

Category:Data Structures

A two-dimensional array is a data structure in Java that is an array of arrays. Each element in the array is an array itself. Generally, a two-dimensional array is referred to be columns and rows:

Initializing a Two-Dimensional Array

int[][] myArray = new int [3][4]; - This initializes a 2D array with 3 rows and 4 columns.
The new keyword is used to allocate memory space for a new 2D array.
Using an initializer list: int[][] myArray = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} };

Rows are the elements in an array that it can store horizontally. row size equal to 2 will create 2 rows. Columns are the elements in an array that it can store vertically.

Examples

Adding Elements in a 2D Array

To add elements to a 2D array, we specify the index values for the row and column where we want to insert the element.

int[][] sampleArray = new int[2][2];

sampleArray[0][0] = 0;
sampleArray[0][1] = 1;

sampleArray[1][0] = 2;
sampleArray[1][1] = 3;

Modifying Elements in a 2D Array

To modify an element in a 2D array, we specify the index values for the row and column we want to modify and assign the new value.

int[][] sampleArray = new int[2][2];

sampleArray[0][1] = 10;

Row-Major Traversal of a 2D Array

Row-major order is one of two common ways to traverse through a 2D array. We access each element in a row before moving on to the next row.

int[][] numbers = { {0, 1, 2}, {3, 4, 5}, {6, 7, 8} };

for (int row = 0; row < numbers.length; row++) {
   for (int col = 0; col < numbers[0].length; col++) {
      System.out.print(numbers[row][col] + " ");
   }

   System.out.println();
}

Output:

0 1 2

3 4 5

6 7 8

Column-Major Traversal of a 2D Array

Column-major order is one of two common ways to traverse through a 2D array. We access each element in a column before moving on to the next column.

int[][] numbers = { {0, 1, 2}, {3, 4, 5}, {6, 7, 8} };

for (int col = 0; col < numbers[0].length; col++) {
   for (int row = 0; row < numbers.length; row++) {
      System.out.print(numbers[row][col] + " ");
   }

   System.out.println();
}

Output:

0 3 6

1 4 7

2 5 8

Traversing 2D Arrays with Enhanced for Loops

When traversing a 2D array with enhanced for loops, the outer loop refers to each row while the inner loop refers to each element in the current row.

int[][] numbers = { {0, 1, 2}, {3, 4, 5}, {6, 7, 8} };

for (int[] currentRow: numbers) {
   for (int value: currentRow) {
      System.out.println(value);
   }
}

Syntax

int[][] test = new int[4][4];